Continuing research on this project is concerned with development of methods which can be used for the synthesis and semisynthesis of peptides under mild conditions, particularly in aqueous solutions within "physiological" ranges of pH and temperature. The thiol-labile bridging methods applied earlier to bidirectional solid phase synthesis of peptides in non-aqueous media are being adapted to work in aqueous media. Specific polymeric reagents which can form reversible covalent linkages with the sidechains of particular amino acids in aqueous solutions are being developed. These will allow attachment of peptide fragments to supporting media during synthesis and will also be useful for separations of peptide mixtures on the basis of sidechain functional groups. Enzyme-labile blocking group for the amino function and for the carboxyl group are being developed and refined so that they can be applied to the sidechain groups of peptide fragments and removed from the sidechain functions of peptides in aqueous solution under mild conditions.